Control valves for a variety of applications have been known for many years. One application is that of pressure regulation wherein it is desirable to control the pressure supplied to a pressure driven device such as an air turbine starter. Most frequently, this is accomplished by means of inlet and outlet pilot signals being directed to a pilot operated valve.
In a common application, the control valve controls the pressure of fluid initially supplied to the air turbine starter to prevent destructive shock from being imparted to the mechanism. As the starter responds, the rate of increase in fluid pressure is typically progressive to effect a smooth, rapid acceleration of the mechanism. In addition, the control valve serves to maintain fluid pressure by responding to fluid pressure sensed on the upstream side of the air turbine starter.
In making a control valve of this type, a problem is to provide combined means for regulating pressure, limiting pressure rise rate, and controlling the speed of the air turbine starter. It is also difficult to meet strict speed requirements over a wide range of load transients. Moreover, control valves usually do not provide high frequency response because of the difficulty in controlling valve dynamics and nonlinearities such as friction and aerodynamic forces.
While overcoming problems of this type, it is also desirable to control the rate of increase in fluid pressure by ramping the fluid pressure set point at a required rate up to a selected maximum. It is also desirable to eliminate the complexity and problems associated with valve systems of this type in sensing pressure derivatives pneumomechanically. Further, it is desirable to eliminate supply pressure sensitivity problems associated with limiting the valve opening rate in conventional control valves.
The present invention is directed to overcoming the above stated problems and accomplishing the stated objects.